TYPHOID FEVER. 371 The center of the superficial colonies is the only portion which shows the yellowish-brown color. The margins of the colony appear somewhat reticulated. The gelatin is not liquefied. Unfortunately, the appearances of the colonies of the Bacillus typhi and the Bacillus coli communis are iden- tical, and make it next to impossible to select a single colony of either with any certainty. The only solution of the problem is to transfer a large number of colonies to some culture-medium in which a characteristic of one or the other species is manifested, and then study the growth; or to grow the colonies upon some special medium in which differences, such as rapidity of growth or acid-production, etc. cause the colonies of the differ- ent species to assume characteristic appearances. A method recently suggested by Eisner1 has materially aided the separation .of these allied bacteria by using a culture-medium upon which the two bacilli develop dif- ferently. The Eisner medium can be made by allowing i kgm. of grated potatoes (the small red German potato is best) to macerate in i liter of water over night. The juice is carefully pressed out, and filtered cold to get rid of as much starch as possible. The filtrate is now boiled and filtered again. The next step is a neutralization, in which Eisner used litmus as an indicator, and added 2.5- 3 c.cm. of a ^ normal solution of sodium hydrate to each 10 c.cm. of the juice. Abbott prefers to use phenol- phthalein as an indicator. The final reaction should be slightly acid. Ten per cent, of gelatin (no peptone or sodium chlorid) is now dissolved in the solution, which is boiled for the purpose, and must then be again neu- tralized to the same point as before. After filtration, the medium receives the addition of i per cent, of potassium iodid. It is filled into tubes and sterilized. When water or feces suspected of containing the ty- phoid bacillus are mixed in this medium and poured 1 Zdtschrift fur Hygiene, xxii., Heft i, 1895 ; Dec. 6, 1896.